This invention relates to the manufacture of pellets, for example, pellets fed to pets and livestock. Typically, the equipment used to make such pellets includes a bin for containing dry pellet material, a motorized feeder, a source of moisture and heat, a motorized conditioner where the moisture is added to the pellett material, and a motorized pellet producing means. This invention relates to equipment in which moisture and heat are supplied with steam and the pellet producing means is a motorized die with holes from which the pellets are extruded.
The current load on the motor of the pellet producing means is a measure of the efficiency of the equipment. This load depends on at least two important factors. First, the load is dependent on the rate at which feed mash is fed into the pellet producing means. The faster this feed rate, the greater the load, Second, the load depends on the composition of the mash, especially its temperature and moisture content.
If these factors do not meet optimum conditions, the die motor will either be used inefficiently if the load is too small or become plugged if the load is too large. For example, up to a certain critical point, moisture, such as steam, acts as a lubricant for the dry material, thereby reducing the current load. Above a certain rate of input of dry material, however, more steam can cause the conditioned mash to thicken, causing the die to become plugged.
In addition to being important to efficiency, accurate inputs of liquids and heat are important to ensuring good quality of pellets. For example, a certain degree of heat during pelleting ensures that the pellets will be digestible.
Two limitations on the ability of reach optimum conditions for both efficiency and high quality production are: current load and heat conditions inside the pellet mill are constantly changing, and the ability to adjust the input of dry material and steam is constrained by the danger of overloading the die motor.
Various methods have been developed to control the relationship during operation of dry feed to steam and other liquid ingredients. Until the 1970's, these ingredients were controlled by operator intervention. In the past two decades however, automatic control systems have been developed for controlling them.
One such control system, disclosed in U.S. Pat. No. 3,932,736 and its improvement patent, U.S. Pat. No. 4,463,430, uses temperature or steam input as an "operating parameter". The system operator selects one of these parameters as a controlling parameter, which then automatically controls the input of ingredients.
Another automatic controls system is disclosed in U.S. Pat. No. 4,327,871. this system regulates temperature by changing the steam input. During operation, the system senses the power consumed by the die motor. After detecting when this power begins to rise as a result of increased steam input, the system then decreases the steam to a point where power consumption is again at a minimum. The same ratio of steam to feed is maintained until power consumption again begins to rise.
In neither of these patents are the inputs of dry material and steam interrelated to maintain a desired current load of the die motor and temperature in the pellet mill. The present invention uses a constant interplay of relationships of the current, temperature, feeder speed, and steam input. The system continually monitors the current and temperature. At periodic intervals, the system determines whether the current is within a tolerance of a predetermined desired value. If the current is not in tolerance, the system adjusts the feeder speed. For each change in feeder speed, the system adjusts the steam input. If the current is within tolerance, the system determines whether the temperature is within tolerance, and if not, adjusts steam input. The amount of each adjustment of feeder speed or steam input is a function of the effect of the next preceding adjustment. The effects of preceding adjustments on the current are also used to predict whether an overload condition is imminent. In this manner, the system not only achieves the optimum load conditions, but also prevents the die from becoming plugged.